[英]Dereferenced union member bytes are not the same
From the tutorial at learnc, I am experimenting with some really basic stuff on pointers and unions. 从learnc的教程中,我正在尝试一些关于指针和联合的非常基础的东西。 In the code below I create a
struct operator
with an anonymous union
consisting of a float
, double
, and int
. 在下面的代码中,我创建了一个带有匿名
union
的struct operator
,该union
包括float
, double
和int
。 Since double
is the biggest one at eight bytes, I expect to see my int
have eight bytes, which it does. 由于
double
是八个字节中最大的一个,因此我希望我的int
具有八个字节,它确实如此。 However, they are not the same bytes as the double! 但是,它们与double字节不同!
typedef enum {
INTEGER = 0,
FLOAT = 1,
DOUBLE = 2,
} operator_type;
typedef struct operator {
operator_type type;
union {
int intNum;
double doubleNum;
float floatNum;
};
} operator_t;
int main() {
operator_t op;
op.type = FLOAT;
op.floatNum = 3.14f;
printf("op.intNum = %i\nop.doubleNum = %f\nop.floatNum = %f\n", op.intNum, op.doubleNum, op.floatNum);
printf("op.intNum [%i, %i, %i, %i, %i, %i, %i, %i, %i]",
&(op.intNum) + 0,
&(op.intNum) + 1,
&(op.intNum) + 2,
&(op.intNum) + 3,
&(op.intNum) + 4,
&(op.intNum) + 5,
&(op.intNum) + 6,
&(op.intNum) + 7,
&(op.intNum) + 8
);
printf("op.doubleNum [%i, %i, %i, %i, %i, %i, %i, %i, %i]",
&(op.doubleNum) + 0,
&(op.doubleNum) + 1,
&(op.doubleNum) + 2,
&(op.doubleNum) + 3,
&(op.doubleNum) + 4,
&(op.doubleNum) + 5,
&(op.doubleNum) + 6,
&(op.doubleNum) + 7,
&(op.doubleNum) + 8
);
return 0;
}
I get the output: 我得到的输出:
op.intNum [-13304, -13300, -13296, -13292, -13288, -13284, -13280, -13276, -13272]
op.doubleNum [-13304, -13296, -13288, -13280, -13272, -13264, -13256, -13248, -13240]
Shouldn't &(op.intNum) == &(op.doubleNum) == &(op.floatNum)
? &(op.intNum) == &(op.doubleNum) == &(op.floatNum)
吗?
Shouldn't
&(op.intNum)
==&(op.doubleNum)
==&(op.floatNum)
?&(op.intNum)
==&(op.doubleNum)
==&(op.floatNum)
吗?
Yes, they should, and they are. 是的,他们应该,而且确实如此。
In order to print an address, use %p
as the format specifier, and cast it to void*
. 为了打印地址,请使用
%p
作为格式说明符,并将其强制转换为void*
。 Read more in How to print variable addresses in C? 在如何在C中打印变量地址中阅读更多内容。
Edit: Why do I have to cast my memory address to (void *)? 编辑: 为什么我必须将内存地址转换为(void *)?
&(op.intNum) + 1
is the address immediately after the end of op.intNum
. &(op.intNum) + 1
是紧接在op.intNum
之后的地址。
That is, if op.intNum is at address A, and sizeof op.intNum is 4, then the expression you wrote has value A+4. 也就是说,如果op.intNum位于地址A,而opof.intNum的大小为4,则您编写的表达式的值为A + 4。
That's a consequence of how pointer arithmetic is defined. 这是如何定义指针算法的结果。
In your case you don't dereference union, for dereferencing you need to declare op as pointer. 在您的情况下,您不取消引用联合,要取消引用,则需要声明op作为指针。
FYI: 供参考:
In your code you print 在您的代码中打印
address of int member, and sizeof(int) is 4 in your architecture, so increment by 1 , your address will be address + sizeof(int) int成员的地址,在您的体系结构中sizeof(int)为4,因此加1,您的地址将为address + sizeof(int)
address of double member and sizeof(double) in 8, in this case each after each increment your address will be address + 8 double成员的地址和sizeof(double)的8,在这种情况下,每次递增后,您的地址将为address + 8
Hope this will help. 希望这会有所帮助。
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